Indoor cellular systems are confronted with a challenging radio propagation environment. The penetration of radio waves is hindered by walls and doors which may define irregular boundaries; numerous obstacles exist such as pipes and ducts which shadow or otherwise restrict radio propagation, and so forth. Serving mobile terminals located inside an office building by means of an outdoor cellular radio base station is usually impractical. While the signal may be able to penetrate exterior walls and windows, the radio signal may not penetrate well into interior spaces.
As a result, efforts have been made to address the particular needs of indoor cellular coverage. A substantial portion of these efforts has been focused around the so-called micro-cell, which is in effect a fully functional cellular base station designed to cover cell sizes on the order of tens or hundreds of square meters.
As with ordinary outdoor macro-cellular systems, each cell in a micro-cellular system may be assigned a group of frequencies. As a mobile terminal moves from one cell to the next, the terminal is handed-off between neighboring micro-cells. The hand-offs are based on received signal strength measurements which are monitored and controlled by a central switch. When the signal strength falls below a predetermined threshold, the terminal is instructed to retune its transceiver to another frequency used by another cell. Various hand-over methods based on received signal strength measurements are known to those skilled in the art.
When the cell size of the micro-cell becomes so small that a user walking down a corridor moves through several cells during a conversation, the number of hand-offs increases substantially, creating a substantial network management problem. One solution to this problem is not to perform a hand-over in the conventional sense, but to keep the terminal operating on the same frequency/time slot combination and to hand-over the downlink from one micro-cell to another.
Such a technique is described in U.S. Pat. No. 4,932,049 entitled "CELLULAR TELEPHONE SYSTEM." Therein is described a system comprising a plurality of contiguous cells each assigned a particular frequency set and having transmitting and receiving means, which are arranged for maintaining continuous communication with mobile terminals moving from cell to cell. Each cell has a plurality of transmitting and diversity receiving sets, or micro-cells positioned at a respective antenna site at the periphery of the cell and configured so that the propagation and reception of signals is limited to substantially the borders of the cell. Control circuitry monitors the strength of each signal received by each of the antenna sites at each frequency channel in the assigned frequency set. Transmission, at each frequency channel in the assigned set, is confined to the antenna set at one sub-site in the cell having the strongest received signal at each frequency.
When a mobile unit moves such that the received signal strength at a subsite other than the one currently transmitting becomes strongest, the system operates to turn off the transmitter at the weaker site and turns on the transmitter at the sub-site at which the stronger signal level is being received. This is known as a so-called virtual handover. Two diversity receiving antennas are also switched to the proper sub-site to receive the call. The frequency does not change and remains as before. Thus the MTSO is not involved and no additional hand-off load is encountered.
One problem with such a system is that the broadcast control channel is transmitted from a single sub-site which covers the entire cell, in an indoor system where sub-sites may be located around corners--or even inside of elevators--the broadcast control channel may not penetrate where the sub-sites are located. Loss of signal to the broadcast control channel leaves the mobile station without essential overhead information. Usually, loss of the broadcast control channel is perceived by the mobile station as a complete loss of signal and forces the mobile station into a reacquisition mode.
A second problem typically associated with micro-cellular systems is that there is little or no time dispersion of signals transmitted within such a system. Time dispersion arises when a reflected signal of significant magnitude arrives delayed in time from the main signal. When the time delay is on the order of the symbol time, intersymbol interference results. Historically, time dispersion has been an undesirable side-effect of radio propagation. However, modern techniques such as MLSE equalization actually advantageously make use of time dispersion to enhance signal reception. Creating time dispersion or the use of macro-diversity have been employed in outdoor cellular and land mobile radio systems such as described in U.S. Pat. No. 5,088,108 entitled "CELLULAR DIGITAL MOBILE RADIO SYSTEM AND METHOD OF TRANSMITTING INFORMATION IN A digital CELLULAR MOBILE RADIO SYSTEM" and U.S. Pat. No. 5,109,528 entitled "HANDOVER METHOD FOR A MOBILE RADIO SYSTEM" both assigned to the instant assignee of the present invention. However, in an indoor system the combined problem of macro-diversity and virtual handover has not been heretofore addressed.
A third problem typically associated with conventional micro-cellular based indoor systems is that both the uplink and downlink are normally served from the same antenna set. When the downlink antenna is selected so is the uplink antenna. This is because existing micro-cellular systems employ narrowband radio receivers. If a wideband radio receiver is used there is no need to restrict reception to a particular antenna set of the signals received from a mobile station. In a non-stationary environment, the uplink and downlink may not be temporally reciprocal and thus it would be advantageous to separately select the uplink and downlink antenna sets. Viz., because the uplink and downlink are at different frequencies, the uplink and downlink channels exhibit different characteristics, such as different levels of Rayleigh fading, etc.